Integrated circuit technology relies on transistors to formulate vast arrays of functional circuits. The complexity of these circuits requires the use of an ever increasing number of transistors. And the more transistors, the more the demand for transistors with faster switching times. As the number of transistors required increases, the total power consumption, the response time and the cost of the circuit also increase. It is desirable then to construct inexpensive transistors which are faster yet more efficient in terms of current gain and switching time.
Silicon has been a standard material for manufacturing transistors for many years. Conventionally, silicon technology has made steady strides in semiconductor evolution by relying strongly on dimensional miniaturization or "scaling" to achieve performance increases. However, miniaturization below today's submicrometer dimensions is difficult to achieve and comes at a prohibitive cost. Therefore, many researchers have decided that homojunction transistors, such as silicon, have reached their performance boundaries. Because of this, the main focus of transistor research has been on SiGe (silicon-germanium) graded base transistors or SiGe heterojunction transistors. Researchers have found additional performance in SiGe type transistors but not without increased time and expense in manufacturing. In fact, SiGe heterojunction bipolar transistors (HBT) have not gained acceptance in the industry due to manufacturing difficulties, bandgap discontinuities and spikes at the emitter-base junction. Instead, graded base SiGe transistors have dominated the current transistor market.
A typical bipolar transistor has a collector, a base and an emitter. When the transistor is activated, a small current is injected into the base of the transistor. The applied bias lowers a constant, built-in energy barrier that blocks the flow of electrons. As the barrier drops, current begins to pass through the transistor, and the device switches to an ON state. The amount of current moving through the device is proportional to, but much larger than, the amount injected into the base. The built-in barrier is created by introducing specific impurities, or dopant atoms, into the silicon when the transistor is fabricated. Doped silicon is known as n-type silicon if it contains an excess of negative charges or p-type silicon if positive charges prevail. The function of a bipolar transistor depends on the electrical properties at the interface between n-type and p-type silicon. An interface between two regions of semiconductor having the same basic composition -silicon, but opposing types of doping is called a homojunction. The joining of two dissimilar materials is known as a heterojunction.
The collector and base of the transistor are normally constructed using doped semiconductor materials. The emitter is usually constructed of a semiconductor material such as n+polysilicon. However, a polysilicon emitter with controlled oxides has a high emitter resistance and works well only at low current levels. At high current levels, the emitter resistance results in a reduction of the effective transconductance. This causes inefficient power consumption and requires more heat dissipation due to the higher series resistance. Metal emitters have been proposed to overcome deficiencies in heterojunction type bipolar transistors. The proposed metal emitters, however, do not contain a semiconductor layer acting as an emitter.
FIG. 1A illustrates the basic problems with modern bipolar transistors 10. Shallow emitter junctions and high surface recombination velocities at the emitter surface 11, result in a large injection current into the emitter 12, low injection efficiencies and consequently low current gain, "beta", and lower power gain at high frequencies. One method of resolving these deficiencies involves reducing the series resistive value of the transistor. This can be accomplished by increasing the transistor's injection efficiency. For example, as shown in FIG. 1B, to overcome the high emitter resistance, a SiGe base HBT 20 can be constructed. This allows for a smaller bandgap material (n.sub.po of SiGe is greater than n.sub.po of Si, where n.sub.po is the electron thermal equilibrium concentration in a p-doped material) to be used for the base which results in a higher injection efficiency. [i.e., n.sub.po /n.sub.po +p.sub.no), where p.sub.no is the hole thermal equilibrium concentration in a n-doped material)] where p.sub.no is from the equation that the efficiency increases when the electron equilibrium concentration becomes increasingly larger than the hole equilibrium concentration. However, SiGe HBT transistor technology has not been proven to be economical nor feasible for actual production.
Higher injection efficiencies in bipolar transistors 30 can also be obtained by using an n+polysilicon emitter contact 31 with a controlled oxide 32 as shown in FIG. 1C. This type of transistor can have high current gain, since the surface recombination velocity at the emitter surface 33 is low. However, the resistance of the emitter 34 is also higher due to the high tunneling barrier height. Another method involves using graded base SiGe transistors. Here, a retrograde Ge concentration is used in the base to create a built-in field which reduces base transit time. However, the complexity and cost of the transistor manufacturing are increased due to the process required to fabricate the graded base material.
Accordingly, what is needed is an inexpensive, easily manufacturable bipolar transistor with an emitter contact having a low tunneling barrier height or "work function". A transistor with such an emitter would have reduced resistivity and, thus, high current gain and fast switching speeds.